Various types of hot dip plated metal sheet have been developed and commercialized up to now. Among these, hot dip galvanized steel sheet is spreading widely as a material for automobiles, buildings, the electric home appliances, etc. due to its superior corrosion resistance and economy.
The present invention may be applied to not only hot dip galvanization, but also aluminum plating, tin plating, and other various types of hot-dip plating bath, but the case of the most general hot dip galvanization system for steel sheet will be taken as an example and explained below.
When continuously producing hot dip galvanized steel sheet, the method of dipping and moving the steel sheet in a hot-dip plating bath to plate it has been generally used.
At this time, it is known that the solid particles of impurities precipitated and deposited at the bottom of the hot-dip plating bath, for example, bottom dross, are stirred up along with the movement of the steel sheet during the plating treatment and stick to the steel sheet plated surface to thereby detract from the appearance of the plated steel sheet.
Various measures are being tried out on the work site to deal with this stirred up bottom dross in hot-dip plating bath, but no complete solution has yet been found.
FIG. 9 shows the outlines of a generally used continuous hot dip galvanization system of steel sheet. In the continuous hot dip galvanization system shown in FIG. 9, after a steel sheet 1 is annealed by an annealing furnace (not shown), it passes through a snout 2 and enters a hot dip galvanization bath 3.
The introduced steel sheet is changed in direction to face upward by a sink roll 4 provided inside the hot dip galvanization bath 3, is corrected for warping by support rolls 5, then is pulled out from a plating bath surface 6.
Next, the two sides of the hot dip galvanized plated steel sheet 1′ are blown with wiping gas from gas wiping nozzles 7 to adjust the amount of coating weight
Furthermore, the plated steel sheet 1′ is passed through a vibration dampening system 8 for correcting its shape and suppressing steel sheet vibration, and then treated to alloy the plating as needed in a galvannealing furnace 9.
Inside the hot dip galvanization bath, Fe elutes from the steel sheet into the hot dip galvanization bath whereby particulate and granular substances comprised of Fe—Zn intermetallic compounds, that is, so-called “dross”, are produced.
In this dross, the part mainly comprised of FeZn7 has a larger specific gravity than the molten zinc, so precipitates and deposits on the plating bath bottom. In general, this is called “bottom dross” (see 10 in FIG. 9).
Bottom dross starts to be stirred up due to the accompanying flow caused by movement of the steel sheet circling the sink roll in the galvanization bath and finally sticks to the surface of the plated steel sheet to cause poor appearance of the plated steel sheet.
In particular, the bottom dross is caught and pressed at the part where the sink roll or support roll and steel sheet contact each other and remains on the plated steel sheet thereby becoming a cause aggravating the poor appearance when press forming the plated steel sheet into the final product.
In particular, on recent work sites, attempts are being made to raise the processing rate of steel sheet to improve production capabilities. Along with this, in the plating bath, the agitation becomes stronger, and the amount of elution of Fe, the cause of formation of dross, increases, and the bottom dross is stirred up much more vigorously.
Further, on the other hand, customers are becoming tougher regarding the quality of appearance of the plated steel sheet they seek. There is therefore pressure on the work floor for solving the problem of stirred up bottom dross.
To solve this problem, various proposals have been made in the past.
For example, Japanese Patent Publication (B2) No. 6-21331 and Japanese Utility Model Publication (U) No. 5-38045 propose a method of suppressing stirred up bottom dross comprising providing a covering plate covering the entire cylinder length of the sink roll and suppressing flow in the plating bath between the sink roll and plating tank bottom and forming a space in which bottom dross deposits below this covering plate.
Further, Japanese Patent Publication (A) No. 6-158253 proposes a continuous hot dip galvanization system providing a multi-hole plate suppressing flow motion in the bath between the sink roll and plating tank bottom.
Furthermore, Japanese Patent Publication (A) No. 2001-140050 proposes a system for prevention of stirred up bottom dross characterized by providing two plate-shaped members of lengths corresponding to 20 to 40% of the sink roll cylinder length away from the sink roll surface from the two ends of the sink roll toward the center.
However, with these proposals, as explained later, it is difficult to completely solve the problem of stirred up bottom dross.
For stirred up bottom dross in hot dip galvanization baths, in the past it had mainly been considered that the tangential direction force caused along with rotation of the sink roll (see 11 in FIG. 9) caused the bottom dross deposited at the bottom near the sink roll to be stirred up.
However, the inventors worked to study the phenomenon of stirred up bottom dross by running 3D flow motion analysis on the inside of hot dip galvanization baths. As a result, they discovered that the flow accompanying the steel sheet becomes strong at the part narrowed by the sink roll.
That is, jet flow occurring at the sides of the contact part of the sink roll moves strongly toward the bottom of the sides in the hot dip galvanization bath, so the inventors discovered that the bottom dross deposited at the bottom of the hot dip galvanization bath was stirred up.
When the steel sheet is a broad width material, as shown in FIG. 10(a), the jet flow occurring at the sides of the contact part of the sink roll causes a force to act on the bottom dross stirring up the bottom dross from near the front center of the sink roll 4 (see A in the figure).
Further, when the steel sheet is a narrow width material, as shown in FIG. 10(b), a force acts on the bottom dross stirring up the bottom dross between the sink roll 4 and the side walls of the hot dip galvanization bath 3 (see B in the figure).
In each case, as a result, the bottom dross is stirred up inside the plating bath in a manner drawing a circle in the vertical direction and enters a floating state. The inventors elucidated the mechanism whereby the bottom dross enters a floating state and has a detrimental effect on plating of steel sheet.
Assuming this mechanism, the prior art has the following problems:
First, the method disclosed in Japanese Patent Publication (B2) No. 6-21331 and Japanese Utility Model Publication (U) No. 5-38045 can effectively prevent stirred up bottom dross by the provision of a covering plate for bottom dross trying to flow in a tangential direction of the circumference due to rotation of the sink roll, but no covering measure or flow regulating measure is taken against the wall surface flow occurring at the two side surfaces of the sink roll, so the stirred up bottom dross cannot be sufficiently suppressed.
Second, the apparatus disclosed in Japanese Patent Publication (A) No. 6-158253 does not provide any means for solving the problem of the wall surface flow occurring at the two side surfaces of the sink roll, so the effect of suppressing stirred up bottom dross is not sufficient.
Further, in the above apparatus, the multi-hole flow regulating plate is provided for substantially the entire sink roll in the width direction, so turbulence occurs between the sink roll and multi-hole flow regulating plate, bottom dross sticks to the surface of the steel sheet not contacting the sink roll, and bottom dross is liable to deposit on the multi-hole flow regulating plate.
Furthermore, in the above apparatus, there is the problem that at the time of replacement of the sink roll etc., the work of threading the steel sheet through the sink roll and rest of the plating apparatus becomes complicated.
Third, in the apparatus disclosed in Japanese Patent Publication (A) No. 2001-140050, two covering plates are set at the two sides of the sink roll in a state separated from each other, so the problem at the time of replacement of the sink roll etc. is solved, but no measure is taken for regulating the wall surface flow occurring at the two side surfaces of the sink roll, so stirred up bottom dross cannot be completely suppressed.
Further, in the above apparatus, when the distance between the sink roll and the plate members is large, the bottom dross deposits on the plate members, so when changing the steel sheet from a narrow width material to a broad width material, the stirring up of the deposited dross is aggravated.
Conversely, when the distance between the sink roll and plate members is small, a strong flow including bottom dross concentrates at this small distance, so the bottom dross is scattered throughout the plating bath and the bottom dross is liable to be caught between the sink roll or support roll and the steel sheet.